Spool valves are commonly employed in a variety of applications to control fluid transport from one or more sources. A typical spool valve consists of a stationary body with a cylindrical cavity machined therein and a spool-like piston for providing for fluid flow control. Such valves are commonly used as “three-way, three-position” and “four-way, three-position” valves, where the number of ways coincide with the number of ports (e.g., an inlet port, one or more outlet ports and an exhaust port) and each of the number of positions is a unique state for the ports (e.g., inlet and outlet ports all closed, only the inlet port is open, or only a outlet port is open). The piston of a spool valve has a cylindrical shape adapted to fit and slide within the cavity of the valve. The stationary valve body includes openings, or ports, each opening having operational states of closed, partially-opened or fully opened. In operation, a piston slides within the cavity to place one port in fluid communication with other such ports. To minimize leaks, the space between the stationary body and the piston is machined to have a tight tolerance, with viscous lubricants added to further minimize inadvertent leakage among ports. Also, spool valves typically use compliant seals, such as O-rings, to further minimize leaks due to imperfections in machining to tight tolerances. But these efforts to reduce leakage introduce friction into the valve, which an actuator must overcome to drive the piston.
Spool valves are designed to control fluid flow with a single actuator. But in practice, the frictional forces require either at least two actuators or a single higher-force actuator to overcome the friction. But using these types of actuators requires higher power consumption, a larger form factor and greater weight than otherwise might be used, some of these factors precluding the use of these types of valves in some classes of products.
To reduce the power and weight of valve actuators, some traditional approaches have integrated shape memory alloys (“SMA”) elements into valves. But nevertheless, the frictional drag exerted by sliding pistons of the spool valves have continued to present an obstacle to increasing the number of valve applications and their performance (e.g., in terms of longevity and reliability). Another drawback to these approaches is that they do not sufficiently compensate for temperature effects on the SMA elements.
In view of the foregoing, what is needed is an improved valve and valve system using SMA actuators to overcome the drawbacks of conventional valves and to adapt SMA actuators to compensate for a broad range of temperatures that otherwise would affect SMA actuator operation.